A problem frequently encountered with grease guns is the entrapment of air in the grease. This entrapped air adversely effects the operation of the grease gun. The patent literature has numerous references to the problem, and a variety of solutions have been proposed.
U.S. Pat. No. 4,298,144 which issued to Pressi in 1981 discloses a grease gun having a tubular casing with a threaded attachment cap. The inner thread of the attachment cap is cut away along a groove running in an axial direction. The groove serves as an air outlet, so that air can escape from the tubular casing through the groove when the attachment cap is not entirely screwed onto the tubular casing. When an air lock is experienced due to entrapped air, the attachment cap is loosened so that continued application of pressure to expel grease expels the unwanted air. The air outlet is made considerably smaller than the grease outlet so that during the expulsion of air as little grease as possible is expelled.
U.S. Pat. No. 5,404,967 which issued to Barry in 1995 discloses a grease gun that has two passageways in which are positioned pistons. The two passageways share a common outlet controlled by a ball valve. The pistons are mounted on a common piston mounting block and, consequently, move in unison. As the pistons move forward in unison, they push a predetermined quantity of grease through the common outlet. If air enters one of the two passageways, grease will continue to pass through the other of the two passageways to the common outlet, causing the ball valve to open and releasing the air lock.
The solution proposed by Pressi is workable and has come into commercial usage. When the grease gun user loosens the attachment cap to open the air passage, care must be taken not to loosen the attachment cap too much. If the attachment cap is too loose it will "pop" off the tubular casing when pressure is applied. When air bleeds off through the air passage, a quantity of grease often follows the air through the air passage making an unwanted mess.
The solution proposed by Barry is a complex and expensive solution to a relatively simple problem. Insufficient time has passed to determine whether the solution proposed by Barry will gain commercial acceptance.